You are recommended to use this before and after playing; so I did. I would say it is effective within about a one-week period. Let me clarify: When your strings are new, it doesn't improve the tactile feeling much (because there isn't much room to improve). After about a week of usage (1hr/day playing, let's say) you realize that sliding from position to position and doing glissandos are pretty decent compared to cleaning the strings by a bare cloth. But then after several days (let's say a week), the strings are just not fresh and the product doesn't do miracles.
Let me also say that in the first couple of uses, you can feel the chemical on your fingers slightly; you may or may not like it. It was okay for me. I welcome the smoothness.
I also don't know how long the effect lasts. The fabric doesn't look dirty at all yet, but the effect has waned after about 20 uses.